1. Field of the Invention
The present invention relates to a two-terminal nonlinear element used for a liquid crystal display device, and a method for fabricating the same. More particularly, the present invention relates to a two-terminal nonlinear element having a layered structure of metal-insulator-metal (MIM structure), and a method for fabricating the same.
2. Description of the Related Art
In recent years, liquid crystal display devices having the features of being thin, lightweight and having low power consumption have been used as display devices for personal computers, wordprocessors, terminals of office automation systems, TV sets, and the like. In these applications, demands for further increasing the capacity and enhancing the image quality of the display have increased.
Conventional liquid crystal display devices generally adopt a single matrix driving method in a TN (twisted nematic) mode or STN (super twisted nematic) mode, such as an averaged voltage driving method. In this method, however, the contrast ratio of the display lowers as the number of scanning lines increases. This method is therefore inappropriate for large-capacity display.
To overcome the above problem, an active matrix driving method has been developed where switching elements are provided for individual pixels constituting a display screen. Thin film transistors or two-terminal nonlinear elements are used as the switching elements for the active matrix driving method. In particular, liquid crystal display devices using two-terminal nonlinear elements as the switching elements are promising because the structure is simple and the production cost is low. Among other types of the two-terminal nonlinear elements, MIM elements having an MIM (metal-insulator-metal) structure have already been commercialized.
The MIM element has nonlinear voltage/current characteristics, where the resistance of the element becomes high when the voltage of an input signal applied to the element is low, and becomes low when the voltage of the input signal is high enough to drive the pixel. The liquid crystal display devices provided with such MIM elements utilize the nonlinear voltage/current characteristics of the MIM elements for on/off switching.
FIG. 10 is a plan view of a liquid crystal display device 50 using conventional MIM elements.
In FIG. 10, a substrate 2, on which MIM elements and signal line terminals 15 connected to the MIM elements are formed (hereinbelow, such a substrate is referred to as a "device-formed substrate"), is located on the back side, and a substrate 3, on which counter electrodes and counter terminals 16 connected to the counter electrodes are formed (hereinbelow, such a substrate is referred to as a "counter substrate"), is located on the front side of the liquid crystal display device 50. The device-formed substrate 2 and the counter substrate 3, which are typically made of glass, are attached to each other with a sealing material 17 interposed therebetween.
The liquid crystal display device 50 is a reflection type monochromatic display device having a so-called H-VGA pixel arrangement of 480 dots (H).times.320 dots (V).
FIG. 11 is a plan view of one pixel formed on the device-formed substrate 2 at an arbitrary position A in a display region 30 of the liquid crystal display device 50 shown in FIG. 10. FIG. 12 is a plan view of the portion of the counter substrate 3 corresponding to the portion of the device-formed substrate 2 shown in FIG. 11. FIG. 13 is a sectional view of the liquid crystal display device 50, taken along line B-B' of FIG. 11.
As shown in FIG. 11, each pixel includes a pixel electrode 7, a signal line 5, and an MIM element 4 formed on a surface 2a of the device-formed substrate 2. A plurality of such signal lines 5 are actually disposed on the device-formed substrate 2 in parallel with each other, and connected to the respective signal line terminals 15 (see FIG. 10) formed at an end of the device-formed substrate 2.
As shown in FIG. 12, a plurality of counter electrodes 9 are formed on a surface 3a of the counter substrate 3 in a stripe shape in a direction perpendicular to the signal lines 5. The counter electrodes 9 are connected to the respective counter terminals 16 (see FIG. 10) formed at an end of the counter substrate 3. The liquid crystal display device having the pixels with the above configuration is driven by applying signals having waveforms, which are determined in accordance with an image to be displayed, to the signal line terminals 15 and the counter terminals 16.
Referring to FIG. 13, each MIM element 4, which is shown as a hatched portion in FIG. 11, is a two-terminal nonlinear element Including a lower electrode 5a as an extension of the signal line 5, an insulating film B formed to cover the lower electrode 5a, and an upper electrode 6 facing the lower electrode 5a via the insulating film 8.
The lower electrode 5a is made of tantalum (Ta), for example, and the upper electrode 6 is made of titanium (Ti), aluminum (Al), or chromium (Cr). The insulating film 8 is made of tantalum oxide (TaO.sub.x), for example.
Referring to FIGS. 14A to 14E, the fabrication process of the MIM element 4 having the sectional structure as shown in FIG. 13 will be described.
A Ta thin firm is first formed on the surface 2a of the device-formed substrate 2 made of glass by sputtering. The Ta thin film is then patterned into a predetermined shape by photolithography to form the signal lines 5 and the lower electrodes 5a. In FIG. 14A, only one lower electrode 5a formed on the surface 2a of the device-formed substrate 2 is shown.
As shown in FIG. 14B, the insulating film 8 made of TaO.sub.x is then formed to cover the patterned signal lines 5 and lower electrodes 5a made of the Ta thin film by anodizing the Ta thin film, for example.
Thereafter, as shown in FIG. 14C, for the formation of the upper electrodes 6, a Ti thin film 6a, for example, is formed on the device-formed substrate 2 covering the lower electrodes 5a and the insulating films 8. A resist 18 made of a photosensitive resin having a shape corresponding to the pattern of the upper electrodes 6 to be formed is applied to the Ti thin film 6a, exposed to light, and developed.
The Ti thin film 6a is then etched to remove the portions thereof which are not covered with the resist 18. The resist 18 is subsequently removed to obtain the upper electrodes 6 having the predetermined shape as shown in FIG. 14D.
A thin film (not shown) made of a transparent conductive material such as ITO (indium tin oxide) is then formed on the resultant substrate covering the upper electrodes 6. The ITO thin film is then patterned into a predetermined shape by photolithography to form the pixel electrodes 7 as shown in FIG. 14E. The pixel electrodes 7 are connected to the upper electrodes 6.
Thus, the MIM elements 4 each having the upper electrode 6 and the lower electrode 5a vertically facing each other via the insulating film 8 are formed.
In the above fabrication process of the MIM elements 4, a total of three photomasks are required to pattern the components into the respective predetermined shapes. These three photomasks are: the photomask used to form the lower electrodes 5a by patterning the Ta thin film; the photomask used to form the upper electrodes 6 by patterning the Ti thin film 6a; and the photomask used to form the pixel electrodes 7 by patterning the ITO film.
Referring back to FIG. 13, an alignment film 10 made of polyimide or the like is formed over the device-formed substrate 2 made of glass with the MIM elements 4 formed thereon, and rubbed in a first direction. Likewise, an alignment film 11 is formed on the counter substrate 3 made of glass with the counter electrodes formed thereon, and rubbed in a second direction which is twisted by 90.degree., for example, from the first rubbing direction of the alignment film 10 on the device-formed substrate 2. The formation and rubbing of the alignment films 10 and 11 are performed to control the orientation of liquid crystal molecules in the liquid crystal layer 14 to be injected in a space between the substrates 2 and 3 after these substrates are attached to each other.
The device-formed substrate 2 and the counter substrate 3 are disposed so that the alignment films 10 and 11 formed thereon face each other, and are attached to each other via the sealing material 17 (see FIG. 10) so that the gap therebetween is kept at about 10 .mu.m. The liquid crystal material is injected in the space between the device-formed substrate 2 and the counter substrate 3 to form a liquid crystal layer 14. Then, the substrates 2 and 3 are sealed to form a liquid crystal cell. Polarizing plates 12 and 13 are disposed on the outer surfaces of the liquid crystal cell, so that the polarizing axes of the polarizing plates 12 and 13 are displaced from each other by 90.degree.. Thus, the liquid crystal display device 50 having the MIM elements 4 for the respective pixels is obtained.
Since the liquid crystal display device 50 is of the reflection type, the polarizing plate 12 disposed on the device-formed substrate 2 is provided with a reflector. Alternatively, a transmission type liquid crystal display device can be obtained by using a transmissive polarizing plate and a backlight. A color display is also possible by forming color microfilters of the counter substrate 3.
The above conventional reflection type liquid crystal display device 50 displays images by reflecting light incident from outside the device. With no backlight required, a thin and lightweight liquid crystal display device with low power consumption can be realized. This type of liquid crystal display device is therefore expected to be commercialized as a display for a portable information terminal, for example. To realize such a display, a reflection type liquid crystal display device with high resolution and large capacity and capable of providing so-called paper-white bright display is required.
However, the reflection type liquid crystal display device is disadvantageous in that, since incident light is partially absorbed by the polarizing plate, the reflectance is normally only 50% or less and thus the brightness is not satisfactory.
To overcome the above disadvantage, there is proposed a liquid crystal display device which has no polarizing plate to effectively use all the incident light. One example of such a liquid crystal display device is a phase transition guest-host type liquid crystal display device.
FIG. 15 is a plan view of a device-formed substrate 2 of a liquid crystal display device 70 of the phase transition guest-host type; FIG. 16 is a plan view of a counter substrate 3 of the liquid crystal display device 70; and FIG. 17 is a sectional view of the liquid crystal display device 70, taken along line C-C', of FIG. 15.
The liquid crystal display device 70 has the HVGA pixel arrangement and the plan view of the liquid crystal display device 70 is basically the same as that of the liquid crystal display device 50 shown in FIG. 10. The description thereof is therefore omitted here. The components of the liquid crystal display device 70 corresponding to those of the liquid crystal display device 50 shown in FIGS. 10 to 14E are denoted by the same reference numerals, and the detailed descriptions thereof are omitted here.
As shown in FIG. 17, the liquid crystal display device 70 has color microfilters 22 formed on the counter substrate 3. Each color microfilter 22 is composed of a cyan microfilter 22a and a red microfilter 22b each of which corresponds to one picture element. Therefore, two picture elements, one for the cyan microfilter 22a and one for the red microfilter 22b, constitute one pixel. Therefore, the number of signal line terminals 15 required for this device is double the number of pixels. Specifically, for a 480 dot display, a total of 960 signal line terminals 15 are disposed on the device-formed substrate 2.
Further, in the liquid crystal display device 70, each pixel electrode 7 serves as a reflector. In other words, the liquid crystal display device 70 has a reflector formed inside the liquid crystal cell, not on the outer surface of the liquid crystal cell as in the liquid crystal display device 50 described above. Aluminum may be used as the material of the pixel electrodes 7 which serve as the reflectors.
In the liquid crystal display device 70, the pixel electrodes 7 are formed on an organic insulating layer 23, unlike the liquid crystal display device 50. As shown in FIG. 17, the surfaces of the portions of the organic insulating layer 23 on which the pixel electrodes 7 are to be formed are made uneven (in FIG. 15, such uneven portions are shown by large and small solid circles). The uneven surfaces of the insulating layer 23 a effects the pixel electrodes 7 which are disposed on the insulating layer 23, making the surfaces of the pixel electrodes 7 uneven. These uneven surfaces of the pixel electrodes 7 serve as light-diffusing and reflective surfaces with high reflectance, which effectively improve the brightness and contrast of the resultant liquid crystal display device. The upper electrodes 6 of the MIM elements and the pixel electrodes 7 are electrically connected via contact holes 19 formed through the organic insulating layer 23.
As shown in FIG. 17, a guest-host liquid crystal layer 40 containing liquid crystal molecules 20 and dichromatic dye molecules 21 is formed between the device-formed substrate 2 and the counter substrate 3. The on/off functionality of the display is performed by applying a voltage across the pixel electrodes 7 formed on the device-formed substrate 2 and the counter electrodes 9 formed on the counter substrate 3 to control the orientations of the liquid crystal molecules 20 and the dichromatic dye molecules 21 contained in the guest-host liquid crystal layer 40.
More specifically, the dichromatic dye molecules 21 orient themselves along the liquid crystal molecules 20. When a voltage is applied, therefore, the liquid crystal molecules 20 and the dichromatic dye molecules 21 orient themselves in a direction substantially perpendicular to the surfaces of the substrates 2 and 3. This allows light incident on the counter substrate 3 to pass through the guest-host liquid crystal layer 40 without being absorbed by the dichromatic dye molecules 21. The light is then reflected by the pixel electrodes 7 serving as reflectors, passes again through the liquid crystal layer 40, and is output from the counter substrate 3.
On the contrary, when no voltage is applied, the liquid crystal molecules 20 and the dichromatic dye molecules 21 are arranged randomly in the liquid crystal layer 40. The incident light is therefore blocked by being absorbed by the dichromatic dye molecules 21.
Thus, a bright display (white display) is obtained when incident light is reflected, while a dark display (black display) is obtained when incident light is blocked. Since no polarizing plate is used, a bright display screen is obtained.
The above conventional liquid crystal display devices have the following disadvantages. The insulating film 8 within each MIM element 4 is as thin as 500 to 700 .ANG.. Since the withstanding voltage of such a thin insulating film is low, breakdown can easily occur due to static electricity generated during the fabrication process. Once such breakdown occurs, the upper electrode 6 and the lower electrode 5a of the MIM element 4 are short-circuited, making the MIM element 4 inoperative as a switching element. The pixel corresponding to such a defective MIM element 4 manifests itself as a point defect, thereby lowering the display quality. If such a defect occurs often, the production yield of the liquid crystal display device decreases undesirably.
In particular, in liquid crystal display devices used as portable information terminals which require high resolution and large capacity, the number of pixels and thus the number of MIM elements to be formed is huge. Further, in the color display, the number of MIM elements to be formed is larger than that required for the monochromatic display with the same number of pixels since, in the color display, each pixel is composed of a plurality of picture elements corresponding to respective color microfilters. Moreover, with the color microfilters, point defeats are colored, making them more conspicuous and thus further lowering the quality of the displayed images. It is therefore important to suppress such point defects.
To prevent the generation of static electricity which is a cause of point defects, various types of environmental control are performed during the fabrication process of liquid crystal display devices, including thorough control of humidity in the fabrication process, grounding of workers, and installation of an ion shower. However, the breakdown of MIM elements due to the generation of static electricity has not been completely eliminated.
To overcome the above problem, improving the resistance of the MIM elements has also been examined.
In the conventional MIM element, not only the portion of the insulating film formed on the top flat surface of the lower electrode, but also the peripheral portion thereof formed on the periphery of the lower electrode are used to constitute the MIM element. Such a peripheral portion of the insulating film tends to intensively receive the electric field. Also, the insulating film may fail to cover the periphery of the lower electrode completely due to its insufficient step coverage. Due to these reasons, the breakdown of the insulating film may easily occur at the periphery of the lower electrode, making the MIM element inoperative.
To overcome the above problem, the peripheral portion of the insulating film may be covered with an intermediate insulating layer. With this additional formation of the insulating layer, only the portion of the insulating film formed on the top flat surface of the lower electrode is used to constitute the MIM element, so that the possibility of insulation breakdown of the MIM element can be reduced. Such a MIM element is disclosed in Japanese Laid-Open Patent Publication Nos. 1-270027, 1-283524, and 3-296024, for example. The MIM elements disclosed in these publications are all formed by forming a lower electrode, an insulating film, an intermediate insulating layer, and an upper electrode in that order. Alternatively, the intermediate insulating layer may be formed prior to the formation of the insulating film.
The insulating film and the intermediate insulating layer may be made of the same material or different materials. When they are made of different materials, impurities may enter the insulating film from the intermediate insulating layer, resulting in a degradation of the MIM element.
When the insulating film which constitutes the MIM element and the intermediate insulating layer are made of different materials or formed in different processes, the adhesion between the intermediate insulating layer and the underlying film decreases, which may cause a peeling of the intermediate insulating layer from the underlying film. For example, when an SiO.sub.2 film is formed as the intermediate insulating layer on the periphery of the lower electrode made of Ta, and then the insulating film is formed on the top flat surface of the lower electrode by anodic oxidation, the previously-formed SiO.sub.2 intermediate insulating layer may peel off partially. This makes it difficult to form the complete intermediate insulating layer as a protection film.
Accordingly, the intermediate insulating layer is preferably formed after the formation of the insulating film. Also, the insulating film and the intermediate insulating layer are preferably made of the same material.
However, when the intermediate insulating layer is formed after the formation of the insulating film, the characteristics of the already-formed insulating film may be degraded if a high temperature is used in the formation of the intermediate insulating layer. This results in degrading the characteristics of the resultant MIM element. More specifically, the intermediate insulating layer made of an oxide or a nitride is generally formed by plasma CVD or reactive sputtering. These film formation techniques generally involve a heating process requiring a comparatively high temperature of about 300.degree. C. Such a high temperature degrades the characteristics of the already-formed insulating film and thus adversely affects the resultant MIM element, causing abnormalities in the characteristics of the MIM element.
FIG. 18 is a graph showing an example of the degradation in the characteristics of an MIM element due to heating. In other words, how the baking temperature, which is performed later than the formation of the insulating layer of the MIM element, influences the voltage/current characteristics of the resultant two terminal nonlinear element is shown. In this graph, curves 1-4 shows the characteristics of the MIM element due to heating when the MIM element was baked at different temperatures after the formation of the upper electrode. Curve 5 shows the characteristics of the MIM element obtained when the baking treatment was not performed.
In general, the voltage/current characteristics of a two-terminal nonlinear element conforms to the Poole-Frenkel current represented by equation (1) below: EQU ln (I/V)=ln A+B.sqroot.(V) (1)
wherein I denotes the current and V denotes the voltage. Coefficient A denotes the electric conductivity of the MIM element and Coefficient B denotes the nonlinearity of the resistance of the MIM element. As coefficient A increases, the resistance of the MIM element is smaller and as coefficient B increases, the steepness of the voltage/current characteristics at and around the threshold voltage is larger, allowing the resultant liquid crystal display device to obtain high contrast. That is, it is desirable to increase coefficient B to enhance the image sharpness.
As shown in FIG. 18, however, coefficient B becomes small when the MIM element is baked after the formation thereof, and further decreases as the baking temperature increases. This decrease in coefficient B is undesirable in the characteristics of the MIM element.
It is preferable, therefore, to form the intermediate insulating layer at a temperature as low as possible. This formation of the intermediate insulating layer at a low temperature is also required when a plastic material having a low heat resistance is used as the substrate.